Being overweight or obese is known to increase your chances of developing serious illnesses such as type 2 diabetes, heart disease and cancer, and maintaining a healthy BMI (Body Mass Index) is one of the best ways to prevent these health issues later in life.

But a growing body of research suggests some people who are classed as "obese" have a better chance of surviving the chronic diseases that obesity causes in the first place. This phenomenon has been labelled the "obesity paradox" and it has researchers puzzled.

Breaking down the BMI

The BMI scale is used worldwide and is considered the best way to measure a person's body fat in relation to their height.

But many experts, including Professor John Dixon, head of clinical obesity research at Baker IDI Heart and Diabetes Institute, warn BMI isn't the most accurate way of telling if someone is a healthy weight, because it will change depending on the person, their age and health issues.

"Optimal weight for health, longevity and mortality isn't constant  and why should it be for all adults of any age and with any disease?" he says.

When bigger is better

The evidence for the obesity paradox has found those who are overweight or obese are more likely to survive renal failure, heart failure, diabetes and coronary heart disease, regardless of their age.

But despite all the recent research into the obesity paradox, experts still don't really know why it occurs.

However, Dixon says, "it certainly is a very real thing". He argues it's easier to understand how older people in particular are protected by carrying extra weight, especially those who suffer serious wasting diseases.

"It's been found that in people with all forms of cancer, the bigger they are, the better they do in treatment, because they have greater reserves of muscle and fat at a time when they are under stress," he explains.

For example, a new review by Deakin University, which looked at research carried out on 20,000 people over the age of 65, found those with a BMI between 23 and 33 actually live longer than people with a BMI in the lower end of the normal weight category (between 18.5 and 23).

But what is harder to understand is why young people with lower BMIs have a smaller chance of surviving a sudden serious illness than those of a similar age who are obese, explains Joseph Proietto, professor of medicine at the University of Melbourne.

"Let's say there's someone with a BMI of 19 who's fit and healthy and someone with a BMI of 34," he says.

"Each has a heart attack because they've inherited high cholesterol. Who'll survive better? The evidence suggests the person with the BMI of 34 has a better chance."

Proietto says it could be because people who are obese are often already being treated for conditions such as high blood pressure and cholesterol.

People whose weight is not an issue on the other hand, probably aren't even aware that they have some of these health problems, giving them a disadvantage if they suddenly fall ill.

Cleaning up the confusion

The obesity paradox seems a huge contradiction to everything we've been told about maintaining a healthy weight. So it begs the question: "why bother?" when it comes to diet and exercise.

It also doesn't help that researchers can't explain why obese people seem to fare better in illness. Until more research is done, they're really working on theories.

But Dixon stresses the obesity paradox is definitely not an invitation to throw healthy eating and exercise habits out the window. Preventing these diseases should always be your priority and the best way to do that is by staying lean when you're young.

"People under the age of 40 who have no illness should really be trying to keep their weight down, because there's no doubt that overweight and obesity is associated with disease: diabetes, heart disease, heart failure," he says.

Up until middle age you should attempt to stay within a BMI of 20-25 and lead a healthy, active lifestyle, Proietto says.

"But as you get older, you shouldn't fuss too much if you gain a little weight, because it's when you're older that you're going to have a stroke or heart attack or surgery," he says.